Air separation is the most effective method for cleaning shredded paper out of glass cullet, a situation which often arises in single-stream recyclable processing plants. Yet modern air separation devices suffer from several potential hazards. First, there is a potential for material to bridge, clog, or otherwise jam the suction chamber, causing downtime. Second, a lack of dwell time within the separation area results in reduced separation efficiency and increased sensitivity to material surge and layering. Third, a sensitivity to material orientation, rather than to specific gravity, causes a reduction in separation efficiency. A final hazard is that some material, especially wet material, tends to stick together as a composite and not be separated.
The first type of hazard typically occurs in suction chambers, where material is dropped through a flow of air produced by a vacuum. These types of devices usually have chutes, plates, or other devices to contain and restrict the airflow, because air takes substantial energy to move in high volumes. Straight drop-out chambers and zig-zag chambers are susceptible to this hazard. Because the cross-sectional area of the separation chamber is small, the device can jam on large items or “divers” (e.g. rebar and sticks) in the fines fraction. The large items bridge the chamber and other items build up on top of the bridge. Potential throughput is also small due to the small chamber.
In response, “air knife” style air separators were developed. Rather than pull the light components (e.g. shredded paper) through a vacuum system, a blower is used to generate a knife of air that pushes the light components forward while the heavy components (e.g. glass) fall through, as for example, in Westeria, Nihot and Walair drum-style separators. While this largely solves the jamming and throughput issues that exist with suction chambers, these air knife separators suffer from the second and third hazards described above. Because it uses only a thin knife of air, this type of machine has a single opportunity to separate the materials, rather than the several seconds or bounces that occur in suction chambers. Thus a large item, such as a rock, on top of a light item, such as a piece of paper, will push the light item through the knife of air and improperly sort it into the heavy components. This effect is particularly acute during “surges” of material, where a large clump of items will bind together and act like a heavy component as the air knife is not strong enough to push it in the desired direction. For the same reason, when wet or sticky materials clump together the air knife cannot separate them due to the absence of material agitation.
Further, the air knife concept acts not as a specific gravity separator, but as a separator based on the ratio of weight to cross sectional area of the item perpendicular to the air flow. For example, a large, flat, heavy item, such as a plate, can either catch the air stream on its broad face and be pushed over or “Frisbee” on top of the air stream, reducing separation efficiency. In a mixed waste stream, items are of extremely variable shape, reducing the separation efficiency of air knives.
FIGS. 7, 8 and 9 depict other prior art approaches that introduce the material in the middle part of a rotating drum. These devices, however, have several of the same disadvantages described above. First, because the material is introduced mid-drum, the light fraction is in immediate contact with the heavy fraction within the drum, allowing the light fraction to either stick to the heavy fraction or wrap around the heavy fraction, causing the light fraction to be improperly sorted. Second, the air circulation is open, which blows potentially solid waste particulate that may contain bio-hazard (bio-aerosols) and other hazards such as glass dust into the atmosphere. Third, the drum operates as the sole separation device such that once material leaves the upper part of the drum, it is no longer sorted. This limits the efficiency by not giving the apparatus a chance to correct a mis-sorted fraction, and by not proving a way of sorting for a medium fraction.
Another sorting strategy is a suction hood placed above a material stream. While this solves the jamming issue of a separation chamber, it tends to be of low efficiency for same reasons recited above for air knives.
Thus there is a need in the art for a sorting apparatus which addresses the above issues.